1. Field of Invention
The present invention relates to a piezoelectric resonator. More particularly, it relates to a piezoelectric resonator in which four external electrodes are disposed on the outer bottom surface of a package, and a piezoelectric oscillator which utilizes the piezoelectric resonator.
2. Description of Related Art
Piezoelectric resonators are extensively employed in various electronic equipment including a communication apparatus, an electronic computer, etc. Typically, the piezoelectric resonator has a structure in which a piezoelectric resonator element made of a piezoelectric material, such as quartz, is accommodated in a package. The piezoelectric resonator has been made smaller and thinner in correspondence with the smaller and thinner construction of electronic equipment. The surface mount type of piezoelectric resonator has come into wide use in recent years. The piezoelectric resonator of the surface mount type includes external electrodes which are joined to patterns disposed on the base plate of the electronic equipment, on the outer bottom surface of the insulating package which is made of ceramics or the like.
Heretofore, a piezoelectric resonator of surface mount type (hereinbelow, “piezoelectric resonator”) has been such that a pair of connection electrodes to be disposed for a piezoelectric resonator element are formed on one side of the piezoelectric resonator element in the lengthwise direction thereof and that the piezoelectric resonator element is mounted in a package in the shape of a cantilever. The piezoelectric resonator has been such that external electrodes are formed at both the end parts of the package in the lengthwise direction thereof so as to prevent any electrical directivity ascribable to mounting, when it is mounted on a substrate. Also, the connection electrodes of the piezoelectric resonator element are electrically connected to the external electrodes. See JP-A-7-74581.
There has also been a related-art piezoelectric resonator wherein external electrodes are formed at the four corners of the outer surface of the bottom of a package, respectively, and the connection electrodes of a piezoelectric resonator element are electrically connected to one pair of external electrodes lying on the diagonal line of the outer bottom surface, respectively. See JP-A-11-214950. The four external electrodes are joined to patterns disposed on the substrate, thereby to enhance the mounting intensity of the piezoelectric resonator element on the substrate. In a case where the cover member of the package is made of a metal, the other pair of external electrodes to which the piezoelectric resonator element is not connected are connected to the ground of the substrate and are electrically connected to the cover member, thereby to electromagnetically shield the interior of the package.
FIG. 10 is a drawing showing a related-art piezoelectric resonator provided with four external electrodes, wherein A is a schematic of the piezoelectric resonator with a cover member omitted therefrom, B is a sectional schematic taken along plane B—B in A, in a state where the piezoelectric resonator includes the cover member and C is a bottom view.
As shown in FIG. 10A, the piezoelectric resonator 10 has a piezoelectric resonator element 14 accommodated in a package 12. The piezoelectric resonator element 14 is made of a piezoelectric material, for example, an AT-cut quartz plate and is provided with excitation electrodes 16a, 16b (the excitation electrode 16b is not shown) at the middle parts of upper and lower surfaces. The piezoelectric resonator element 14 has a pair of connection electrodes 18 (18a, 18b) on one side in the lengthwise direction thereof. The respective connection electrodes 18 are formed in continuation to the upper and lower surfaces of the piezoelectric resonator element 14 and are electrically connected to the corresponding excitation electrodes 16 (16a, 16b), and the electrodes of the upper and lower surfaces are formed in symmetry.
Inside the package 12, a pair of mount electrodes 20 (20a, 20b) are disposed in correspondence with the connection electrodes 18. As shown in FIG. 10B, the corresponding connection electrodes 18 are secured to the mount electrodes 20 by a conductive adhesive 22. The piezoelectric resonator 10 has external electrodes 24 (24a–24d) at the four corner parts of the outer surface of the bottom of the package 12, respectively. One mount electrode 20a in the package 12 is electrically connected to the external electrode 24a through a through-hole not shown. The other mount electrode 20b is electrically connected to the external electrode 24c through a connection wiring portion 26 shown in FIG. 10A (refer to FIG. 10C). That is, the pair of electrodes of the piezoelectric resonator element 14 are electrically connected to one pair of external electrodes 24a, 24c lying on the diagonal line of the package 12. The other pair of external electrodes 24b, 24d are connected to the ground of a substrate and are electrically connected to a cover member 28 so as to electromagnetically shield the interior of the package 12. The piezoelectric resonator 10 has the upper end of the package 12 sealed by the cover member 28 as shown in FIG. 10B.